1. Field of the Invention
The present invention relates to a signal receiver, particularly to a low voltage differential signal receiver.
2. Description of the Related Art
The requirement for high speed data transmission is pushing the interface technology toward the high-speed, serial, differential, low power consumption, and point-to-point interface. The LVDS (Low Voltage Differential Signal) technology exactly has all the features a high speed data transmission needs. LVDS is a universal transmission protocol and has been extensively used in the systems requiring the integrity, low-jitter, and common-mode characteristics of signals, especially in the high-speed data transmission in communication systems and display interfaces.
A transmission interface usually needs many sets of LVDS transmitters, and each set of LVDS needs a receiver to receive and amplify weak differential signals. Then, the differential signals are transmitted into a chip for further signal processing.
A common LVDS receiver uses only two NMOS (Negative-channel Metal-Oxide Semiconductor) or PMOS (Positive-channel Metal-Oxide Semiconductor) transistors to sense differential signals and is hard to completely receive the differential signals having a higher or lower common mode voltage. An “LVDS I/O Interface for Gb/s-per-pin Operation in 3.5 um CMOS”, which was published in IEEE journal of Solid State Circuits, disclosed an LVDS receiver using a preamplifier to receive and amplify weak differential signals and using a regenerative circuit to pull up the differential signals to a full range. Each preamplifier needs an additional bias circuit, and the bias circuits consume a large proportion of power.
In a high speed data transmission circuit, the transmitter transmits signals to the receiver at another side via transmission wires. When the transmitter shuts down and no more outputs signals, the receiver will receive over 20 mV differential noise via the transmission wires, which will interfere with the operation of the recipient-side circuit.
In a common fail-safe circuit, a voltage-division resistor generates a tiny voltage difference to function as the offset voltage of the recipient side to overcome differential noise. Refer to FIG. 1. A pull-up resistor RPU and a pull-down resistor RPD are arranged before a conventional LVDS receiver 1 and respectively connected to two transmission wires to perform a voltage division function. Such a fail-safe circuit, which uses resistors to divide voltage and form offset voltage, has a drawback that the circuit needs additional resistors to divide voltage. If the voltage-division resistors are realized in a printed circuit board, the complexity and cost of the printed circuit board will increase. The conventional fail-safe circuit also has a drawback that the voltage-division resistors can only generate unidirectional compensation. Thus, the positive input terminal of the receiver always has an offset voltage greater than the negative input terminal, which results in the signal duty-cycle drift at the output side of the receiver, especially at a low-frequency signal transmission.